In the medical field, there have widely been used radiographic image capturing apparatus that apply radiation to a subject and guide radiation that has passed through the subject to a radiation conversion panel, which captures a radiographic image from such radiation. Known forms of radiation conversion panels include a conventional radiation film for recording a radiographic image by way of exposure, and a stimulable phosphor panel for storing radiation energy representing a radiographic image in a phosphor, and reproducing the radiographic image as stimulated light by applying stimulating light to the phosphor. The radiation film with the recorded radiographic image is supplied to a developing device to develop the radiographic image. Alternatively, the stimulable phosphor panel is supplied to a reading device to read the radiographic image as a visible image.
In an operating room or the like, it is necessary to read a recorded radiographic image immediately from a radiation conversion panel after the radiographic image has been captured for the purpose of quickly and appropriately treating a patient. As a radiation conversion panel that meets such requirements, there have been developed a radiation conversion panel of a direct conversion type having a solid-state detector for converting radiation directly into an electric signal, and a radiation conversion panel of an indirect conversion type having a scintillator for temporarily converting radiation into fluorescence and a solid-state detector for converting the fluorescence into an electric signal.
A radiation conversion panel of the direct conversion type or the indirect conversion type, a controller for controlling the radiation conversion panel to read a radiographic image therefrom as an electric signal, a communication unit for sending signals including the electric signal to an external circuit, and a power supply are encased in a panel housing unit, thereby making up a radiographic image capturing apparatus, which also is referred to as an electronic cassette. The electronic cassette is thicker and heavier than a radiographic image capturing apparatus that incorporates a stimulable phosphor panel therein.
In order to make the electronic cassette as light and thin as a radiographic image capturing apparatus that incorporates a stimulable phosphor panel therein, it is desirable to reduce the thickness of the panel housing unit as much as possible, and also to construct the radiation conversion panel from lighter components. For example, if a radiation conversion panel of the indirect conversion type comprises a stacked assembly made up of a board, a signal output layer disposed on the board for outputting a radiographic image as an electric signal, a photoelectric transducer layer for converting fluorescence into the electric signal, and a scintillator, then the board may be non-vitrified, i.e., the board may be made of plastic rather than glass, and the signal output layer may be constructed from TFTs (Thin Film Transistors) made of an amorphous oxide semiconductor.
However, since plastic has a greater coefficient of thermal expansion than glass, plastic tends to be deformed under environmental conditions, e.g., temperature, humidity, etc., of the radiation conversion panel.
Japanese Patent No. 2706725 discloses that a thermal strain corrector, which has the same coefficient of thermal expansion as a photoelectric transducer layer, is bonded to a surface of a board disposed oppositely to the photoelectric transducer layer, thereby minimizing deformation (warpage) of the board.
According to the technology disclosed in Japanese Patent No. 2706725, since the thermal strain corrector and the board, which have widely different coefficients of thermal expansion, are bonded to each other, the interface therebetween is stressed due to repeated temperature changes, which tends to cause the assembly to crack or peel. More specifically, since the thermal strain corrector simply is bonded in a planar form to the radiation conversion panel, in a case where the radiation conversion panel is deformed due to temperature changes, the radiation conversion panel is likely to crack or peel. It is difficult to prevent the radiation conversion panel from being deformed due to temperature changes.
Since plastic has a greater coefficient of thermal expansion than glass and tends to be deformed under environmental conditions, e.g., temperature, humidity, etc., of the radiation conversion panel, plastic fails to provide sufficient adhesion between the photoelectric transducer layer and the scintillator of the radiation conversion panel.
Japanese Laid-Open Patent Publication No. 09-054162 discloses that adhesion between a scintillator and a photoelectric transducer layer is achieved by performing adhesive bonding therebetween. Japanese Laid-Open Patent Publication No. 09-257944 reveals that increased adhesion is achieved by depressurizing the cassette.
However, adhesive bonding is problematic in that due to the different coefficients of thermal expansion of various components used in the radiation conversion panel, the bonded surfaces (interfaces) are stressed due to repeated temperature changes, which also tends to bring about cracking or peeling in the radiation conversion panel. Depressurization of the cassette requires an evacuating system including a vacuum pump and other parts, resulting in increased system size and cost.
In an electronic cassette, radiation is applied through a subject to the panel housing unit in order to capture a radiographic image of the subject. If in the electronic cassette, the radiation conversion panel is held closely against an inner wall surface of the panel housing unit without any gap therebetween, then the radiation conversion panel can be brought in close proximity to the subject, thereby reducing image blurs of the radiographic image, and the electronic cassette can be manufactured with a low profile. From the standpoint of reworking (recycling) and maintenance of the radiation conversion panel, the radiation conversion panel should preferably be pressed against and secured to the inner wall surface, rather than being bonded thereto. The panel housing unit should preferably be of an integrally molded seamless structure for preventing ambient light from entering into the panel housing unit, so as to provide a desired light blocking capability.
Japanese Laid-Open Patent Publication No. 2002-311526 discloses that a unit including a radiation conversion panel is movable into and out of a casing along rails that are formed on an inner wall of the casing.
If the panel housing unit is made of CFRP (Carbon-Fiber-Reinforced Plastics) or the like to provide better load resistance and reduced weight, then if the radiation conversion panel is taken into and out of the panel housing unit while the radiation conversion panel is held in contact with the inner wall surface of the panel housing unit, some of the carbon fibers that constitute the CFRP may become broken and frayed, which tends to degrade the quality of the captured radiographic image. Furthermore, if the radiation conversion panel is taken into and out of the panel housing unit while the radiation conversion panel is held in contact with the inner wall surface of the panel housing unit, the surface of the radiation conversion panel, e.g., the surface of the scintillator, is likely to become damaged. In the case where the surface of the radiation conversion panel is damaged, the quality of the captured radiographic image is degraded, and control lines for supplying control signals to read the radiographic image, as well as signal lines for outputting signals representative of the radiographic image to an external circuit, are likely to be cut off.